posted on 2017-08-24, 00:00authored byJaime de Anda, Ernest Y. Lee, Calvin K. Lee, Rachel R. Bennett, Xiang Ji, Soheil Soltani, Mark C. Harrison, Amy E. Baker, Yun Luo, Tom Chou, George A. O’Toole, Andrea M. Armani, Ramin Golestanian, Gerard C. L. Wong
Bacteria exhibit surface motility
modes that play pivotal roles
in early-stage biofilm community development, such as type IV pili-driven
“twitching” motility and flagellum-driven “spinning”
and “swarming” motility. Appendage-driven motility is
controlled by molecular motors, and analysis of surface motility behavior
is complicated by its inherently 3D nature, the speed of which is
too fast for confocal microscopy to capture. Here, we combine electromagnetic
field computation and statistical image analysis to generate 3D movies
close to a surface at 5 ms time resolution using conventional inverted
microscopes. We treat each bacterial cell as a spherocylindrical lens
and use finite element modeling to solve Maxwell’s equations
and compute the diffracted light intensities associated with different
angular orientations of the bacterium relative to the surface. By
performing cross-correlation calculations between measured 2D microscopy
images and a library of computed light intensities, we demonstrate
that near-surface 3D movies of Pseudomonas aeruginosa translational and rotational motion are possible at high temporal
resolution. Comparison between computational reconstructions and detailed
hydrodynamic calculations reveals that P. aeruginosa act like low Reynolds number spinning tops with unstable orbits,
driven by a flagellum motor with a torque output of ∼2 pN μm.
Interestingly, our analysis reveals that P. aeruginosa can undergo complex flagellum-driven dynamical behavior, including
precession, nutation, and an unexpected taxonomy of surface motility
mechanisms, including upright-spinning bacteria that diffuse laterally
across the surface, and horizontal bacteria that follow helicoidal
trajectories and exhibit superdiffusive movements parallel to the
surface.